A variety of electrochemical cells falls within a category of cells often referred to as solid polymer electrolyte (“SPE”) cells. An SPE cell typically employs a membrane of a cation exchange polymer that serves as a physical separator between the anode and cathode while also serving as an electrolyte. SPE cells can be operated as electrolytic cells for the production of electrochemical products or they may be operated as fuel cells.
Fuel cells are electrochemical cells that convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. A broad range of reactants can be used in fuel cells, delivered in gaseous or liquid streams. For example, the fuel stream may be substantially pure hydrogen gas, agaseous hydrogen reformate stream, or an aqueous alcohol, for example methanol in a direct methanol fuel cell (DMFC). The oxidant may, for example, be substantially pure oxygen or a dilute oxygen stream such as air.
In SPE fuel cells, the solid polymer electrolyte membrane typically is a perfluorinated sulfonic acid polymer membrane in acid form. Such fuel cells are often referred to as proton exchange membrane (“PEM”) fuel cells. The membrane is disposed between and in contact with the anode and the cathode. Electrocatalysts in the anode and the cathode induce the desired electrochemical reactions and may be, for example, a metal black, an alloy or a metal catalyst supported on a substrate, e.g., platinum on carbon. SPE fuel cells typically also have a porous, electrically conductive sheet material in electrical contact with each of the electrodes that permits diffusion of the reactants to the electrodes. In fuel cells that employ gaseous reactants, this porous, conductive sheet material is sometimes referred to as a gas diffusion backing (“GDB”), which typically is a carbon fiber paper or carbon cloth. An assembly including the membrane, anode and cathode, and gas diffusion backing layers for each electrode, commonly is referred to as a membrane electrode assembly (“MEA”). Flow field plates, made of a conductive material that provide flow fields for the reactants, are placed between adjacent MEAs. A number of MEAs and flow field plates are assembled in this manner to provide a fuel cell stack.
US 2004/0112532 A1 published Jun. 17, 2004 discloses methods for making edge-sealed MEAs. In one embodiment, two annular layers of a thermoplastic material, a catalyst-coated membrane (CCM), and two non-edge-sealed gas diffusion layers (GDLs), in which a peripheral portion of the that does not bear catalyst coatings extends beyond the edges of the GDLs, are bonded together in a one-step process. The published application also describes a two-step process using two annular layers of a thermoplastic, a CCM, one non-edge-sealed GDL, and a second edge-sealed GDL, in which a peripheral portion of the membrane (not bearing catalyst coatings) remains within the edges of the GDLs. In this two-step process the edge-coated GDL is fabricated in a separate prior step. Alternatively, the published application discloses a process wherein an annular scrim layer circles the perimeter of the CCM, the scrim remaining within the edges of GDLs.
Most of the single process step embodiments disclosed in US 2004/0112532 A1 describe the edge of the membrane as extending all the way to the outer edge of the MEA. In this design, the extended portion of the membrane prevents the GDLs from contacting each other, thereby avoiding unwanted electrical pathways. However, such extension of the membrane often is undesirable since fuel cell coolant may come into contact with its exposed edges and thus cause degradation of the MEA.
The published application further discloses use of an annular scrim encircling the perimeter of a membrane that does not extend to the edges of the GDL. Such a construction is difficult to manufacture, however, since care must be taken to avoid inaccuracies in alignment or sizing that could result in gaps between the edges of the annular scrim layer and the edges of the polymer membrane. Such gaps may cause the gas diffusion backing layers to touch one another, resulting in unwanted electrical pathways. Furthermore, molten thermoplastic tends to flow sideways during hot pressing, which may cause the membrane border to move away and separate from the scrim layer. Such separations increase the potential for unwanted electrical pathways.
The published application discloses use of a shim that is substantially the same shape and size as the annular layers of thermoplastic and, therefore, the inner edges of the shim and the annular layers of the thermoplastic are coextensive. A disadvantage of having the inner edges coextensive is that lateral flow of the molten thermoplastic beyond the edges of the shim during the hot pressing step causes non-uniformity in the thickness of the seal area in the proximity of the electrochemically active area.
Accordingly, a need remains for a simple economical method for making UMEAs in small or large quantities, utilizing simple tooling. A need also remains for a fabrication method that is adaptable to a wide variety of different fuel cell designs.